German patent disclosure No. 26 49 669 discloses a burner having a combustion chamber in which a rotating gasifier pot is located in the front region thereof which consists of a bottom and a tubular portion. The output opening of the gasifier pot is located at an axial distance from the front wall of combustion chamber. The tubular portion of the gasifier pot is enclosed at a radial distance by an annular diversion chamber, thus forming an injector channel for the flow of air.
German patent disclosure No. 33 46 431 remarks with respect to the burner disclosed in the previously described German patent disclosure that on a sufficient oversupply of air a good mixture of fuel and combustion air is obtained which is indicated by a blue flame. However, in continuous operation a high oversupply of air is not permissible, because then the CO.sub.2 -value and the combustion efficiency will not meet desired requirements. The additional air disturbs the heat balance in such a way that condensation takes place on the back wall. When operating close to the stoichiometric region insufficient mixing of oil vapors, fresh air and combustion gases takes place. When, for ameliorating the mixing operation, baffles or bypasses were inserted or a change of the injector geometry was made, a lowering of the injector action took place. In turn, this caused reduction of the recirculation of combustion gases. Further, the temperature balance was negatively affected, and condensation effects and an inpermissible high increase of the NO.sub.x and CO gases took place. Accordingly, the German patent disclosure No. 33 46 431 had the object to improve the burner according to the earlier German patent disclosure No. 26 49 669 in such a way that the oil vapor is well mixed with fresh air and combustion gas without negatively affecting the injector action. The improved burner according to the cited German patent disclosure also has a rotating evaporator cup. This cup is closed on the flame side and has an outlet for the evaporated fuel only on the motor side. The evaporator cup is provided with a plurality of rows of holes distributed over the circumference and is surrounded by a annular deflection chamber for the air supply. Gasified fuel and air then flow between the evaporator cup and the flame tube in two concentric flows of annular cross section, strike a baffle ring, mix, and then form a flame. The disadvantage is that the evaporator chamber is not subject to a forceful flow of hot gases, and therefore deposits form there that soon impair the function of the burner. In particular, a major emission of unburned hydrocarbons occurs upon shutoff of the burner.
French patent No. 2 269 029 also discloses a burner having a rotating evaporator cup that is closed on the flame side. The evaporator cup is lined on the inside with a wire mesh, which serves to prevent an outflow of the fuel. This burner needs a strong blower that requires a relatively large amount of energy, because the fresh air and the air and gas mixture are deflected several times. Another disadvantage is that, after shutoff of the burner, a large amount of fuel is still evaporating from the wire mesh, which was previously swept with air and therefore has remained relatively cool; once again, a major emission of hydrocarbons is the result.
In the burner according to the Swiss patent No. 628 724 a so-called mixing tube and a flame tube are provided coaxially to a nozzle. In operation, oil is atomized by the nozzle into the mixing tube into which also the air necessary for combustion is blown. At the distal end of the mixing tube a flame is formed. A part of the hot combustion gases is then recirculated by injector effect back to the inlet of the mixing tube. There it is mixed with the oilmist/air mixture to provide a heat exchange. Thanks to the recirculation of a part of the combustion gases this burner permits a substantial gasification of the oil droplets in the mixing tube. Therefore a relatively good combustion with little soot formation takes place. However, this advantage is at the expense of an increased formation of nitrogen oxides (NO.sub.x). The burner requires a relatively long flame tube. Because an expansion of the flame takes only place after it leaves the flame tube, there exists a relatively large flame zone with high temperature. This facilitates formation of nitric oxides. A further disadvantage of the burner is caused by the fact that on start the mixing tube is cold, and therefore does not cause evaporation. For this reason the flame is very sooty until the mixing tube reaches a high temperature and is in a position to vaporize efficiently the oil droplets hitting it.